The present invention relates to optical fiber diffuser devices for emitting light along a length of the fiber diffuser, and more particularly the invention relates to optical fiber diffusers and methods of producing them with preselected light intensity distribution along the length of the diffuser.
The use of optical fiber as a waveguide to deliver light from a light source to a remote location has long been considered desirable. A number of medical applications, such as photodynamic therapy, interstitial laser photo-coagulation or interstitial laser hyperthermia for tumor destruction, require a diffuser that emits laser light radially from the optical fiber. One of the main challenges of making such a device is to have the light emitted homogeneously along the length of the diffuser tip, especially for tips longer than 5.0 mm. In some applications the fiber diffuser needs to be thin enough to go through hollow-bore needles and endoscopes.
Present cylindrical fiber diffusers use micro-beads or Rayleigh scatters distributed along the fiber tip to scatter the light radially. The amount of light scattered can be controlled by the size and density of microbeads. The diffuser outer diameter range from 0.356 to 1.4 mm (typically 1 mm). U.S. Pat. Nos. 5,196,005 and 5,330,465 issued to Doiron et al. disclose such a diffuser tip having scattering centers embedded in a silicon core that abuts the end of an optical fiber. The scattering centers are embedded in the silicon core in such a way that they increase in density from the proximal end of the diffuser abutting the optical fiber to the distal end of the diffuser. U.S. Pat. No. 5,269,777 issued to Doiron et al. discloses a diffuser tip having a silicon core attachable to the end of an optical fiber. The cylindrical silicone core is coated with an outer silicon layer having scattering centers embedded therein.
U.S. Pat. No. 6,643,253 issued to Beer et al. is directed to an optical fiber diffuser including an attachment that abuts the end of an optical fiber. The diffuser includes a polymeric core in which scattering centers are embedded.
U.S. Pat. No. 4,986,628 issued to Lozbenko et al. teaches an optical fiber diffuser attachment that abuts the end of an optical fiber. The diffuser is made of an optically turbid medium which may be polymer based which is contained in a protective envelope or sheath that slides over the end of the optical fiber.
U.S. Pat. No. 5,207,669 issued to Baker et al. discloses an optical fiber diffuser tip that abuts the end of an optical fiber for providing uniform illumination along the length of the diffuser tip. The diffuser section is produced by thinning the higher refractive index cladding surrounding the multimode fiber core so it has a thickness less than the penetration depth of the evanescent field to permit penetration of the cladding by the evanescent fields along the diffuser section. Some of the light propagating down the fiber core will therefore be emitted and some reflected back into the core at each point along the diffuser tip.
Single mode fibers with weak tilted phase gratings couple light out of the fiber have been disclosed in T. Erdogan and J. E. Sipe, Tilted Fiber Phase Gratings, J. Opt. Soc. Am. A/Vol. 13, No. 2/February 1996.
There are several inherent disadvantages of these types of diffusers including difficulty in achieving illumination homogeneity for long diffusers, and that typically they are non-Lambertian emitters, many are restricted to use at the ends of the optical fiber, and the diffuser tips can break loose at high light intensity as have been observed and they are relatively expensive in that separate diffuser tips have to be produced and adjoined to the end of the optical fiber.
Therefore, there is a need for optical diffusers that approximate Lambertian emission, are not limited to the ends of the fiber and do not require the assembly of separate component parts.
It is an object of the present invention to provide an optical fiber diffuser device that can be produced in any portion of an multimode optical fiber. It is also an objective of the present invention to provide an optical fiber diffuser device that is integrally formed with an optical fiber.
An advantage of the optical fiber diffuser devices constructed in accordance with the present invention is that they can be produced with variable intensity distributions along the length of the diffuser as required for the particular application for which the diffuser is designed. Another advantage of the diffusers is they are not attached to the end of the fiber as a separate piece but are formed anywhere along the optical fiber as part of the fiber itself.
The present invention provides an optical fiber diffuser device. The optical fiber diffuser device comprises a multimode optical fiber having a core and a cladding surrounding the core and including at least one Bragg grating in an preselected length of the core defining a diffuser portion. The at least one Bragg grating includes an effective modulated index of refraction along a length of the diffuser portion for coupling optical radiation radially outwards from the diffuser portion with a preselected intensity distribution as a function of distance along the length of the diffuser portion.
In this aspect of the invention the Bragg grating may be a type II strong Bragg grating. The diffuser emits light with a substantially Lambertian intensity distribution.
The present invention also provides a method of producing an optical fiber diffuser device, comprising providing a multimode optical fiber having a core and a cladding surrounding the core; calculating a coupling coefficient based on a preselected intensity distribution of optical radiation to be radially emitted along a preselected length of the multimode optical fiber and producing at least one Bragg grating in the core of the multimode optical fiber along the preselected length. The at least one Bragg grating has an effective length and including an effective modulated index of refraction along the preselected length of the diffuser portion corresponding to the coupling coefficient for coupling optical radiation radially outwards from the diffuser portion with a preselected intensity distribution as a function of distance along the length of the diffuser portion.
In another aspect of the invention there is provided a device for photothermal treatment of tumors. The device comprises a fiber diffuser including a multimode optical fiber having a core and a cladding surrounding the core. The device includes at least one type II Bragg grating in a preselected length of the core defining a diffuser portion. The at least one type II Bragg grating includes an effective modulated index of refraction along the length of the diffuser portion for coupling light radially outwards from the diffuser portion with a preselected intensity distribution as a function of distance along the length of the diffuser portion. The device includes a light source connected to one end of the multimode optical fiber.
In this aspect of the invention the diffuser portion emits light with a substantially Lambertian intensity distribution.
In another aspect of the invention there is provided a method of monitoring light signals transmitted along a multimode optical fiber. The method comprises forming a Bragg grating in a preselected length of a multimode optical fiber, the Bragg grating having an effective grating strength to couple some of the light transmitted along the multimode fiber out of the multimode fiber. The Bragg grating has an effective modulated index of refraction along the preselected length to give a desired intensity distribution of light emitted radially from the multimode fiber along said preselected length. The method includes detecting light emitted radially from the preselected length of the multimode fiber containing the Bragg grating.
The present invention also provides an optical sensor comprising a light detection means and an optical fiber connected to the light detection means. The optical fiber includes a multimode optical fiber having a core and a cladding surrounding the core and including at least one type II Bragg grating in a preselected length of the core defining a diffuser portion. The at least one type II Bragg grating includes an effective modulated index of refraction along the preselected length of the diffuser portion for coupling a fraction of light incident on the diffuser portion into the core which is transmitted to the light detection means.